The opening was developed by Russian master Alexander Petrov.
The main idea of Petrov's Defense (or so called Russian game) is not to defend e5 pawn, but to symmetrically attack white's e4 pawn. This is strategically rich opening and it's being used by top player around the globe even these days.  

1. e4 e5 2. Nf3 Nf6

White has four main choices for his third move:

3. Nc3, which may transpose to the Four Knights Game or the Three Knights Game
3. Bc4, which may lead to the Boden-Kieseritzky Gambit or transpose to the Four Knights Game
3. Nxe5, the Classical variation
3. d4, the Steinitz Variation
White usually prefers 3. Nxe5, 3. Nc3 or 3. d4.